CHAPTER VII
THE GERMAN TRANSCENDENTALISTS
To complete our historical survey of the morphology of the early 19th century we have now to turn back some way and consider the curious development of morphological thought in Germany under the influence of the Philosophy of Nature. We have already seen many of these notions foreshadowed by Goethe, who had considerable affinity with the transcendentalists, but the full development of transcendental habits of thought comes a little later than the bulk of Goethe's scientific work, and owes more to Kielmeyer and Oken than to Goethe himself.
A great wave of transcendentalism seems to have passed over biological thought in the early 19th century, arising mainly in Germany, but powerfully affecting, as we have seen, the thought of Geoffroy and his followers. Many ideas were common to the French and German schools of transcendental anatomy, the fundamental conception that there exists a unique plan of structure, the idea of the scale of beings, the notion of the parallelism between the development of the individual and the evolution of the race. It is difficult to disentangle the part played by each school and to determine which should have the credit for particular theories and discoveries. The philosophy seems to have come chiefly from Germany, the science from France. It must be borne in mind that German comparative anatomy was largely derivative from French, that the Paris Museum was the acknowledged anatomical centre, and that Cuvier was its acknowledged head.
It is probably correct to say that the credit mainly belongs to the German transcendental school for the law of the parallelism between the stages of individual development and the stages of the scale of beings, and the theory of the repetition or multiplication of parts within the individual. The vertebral theory of the skull is a particular application of the second of these generalisations.
The law of parallelism[141] seems to have been expressed first by Kielmeyer (1793),[142] who gave to it a physiological form, saying that the human embryo shows at first a purely vegetative life, then becomes like the lower animals, which move but have no sensation, and finally reaches the level of the animals that both feel and move.
The idea was next taught by Autenrieth in 1797.[143]
Oken (1779-1851) in his early tract Die Zeugung (1805), and in his Lehrbuch der Naturphilosophie (1809-11) elaborated the thought, and taught that every animal in its development passes through the classes immediately below it. "During its development the animal passes through all stages of the animal kingdom. The fœtus is a representation of all animal classes in time."[144] The Insect, for example, is at first Worm, next Crab, then a perfect volant animal with limbs, a Fly (ibid., p. 542).
As Nature is "the representation of the individual activities of the spirit," so the animal kingdom is the representation of the activities or organs of man. The animal kingdom is therefore "a dismemberment of the highest animal, i.e., of Man" (p. 494). Now "animals are gradually perfected, entirely like the single animal body, by adding organ unto organ"—the way of evolution is the way of development. Hence "animals are only the persistent fœtal stages or conditions of Man," who is the microcosm, and contains within himself all the animal kingdom.
Oken was himself a careful student of embryology; von Baer[145] speaks of his work (published in Oken and Kieser, Beiträge zur vergleichenden Zoologie, Anatomie und Physiologie, 2 pts., 1806-7) as forming the turning-point in our understanding of the mammalian ovum. He had accordingly actually observed a resemblance in certain details of structure between the human fœtus and the lower animals; but the peculiar form which the law took in his hands was a consequence of his hazy philosophy. He saw the relation of teratological to fœtal structure, for he affirmed that "malformations are only persistent fœtal conditions" (p. 492).
The idea of comparing the embryo of higher animals with the adult of lower was widely spread at this time among German zoologists. We find, for example, in Tiedemann's brilliant little textbook[146] the statement that "Every animal, before reaching its full development, passes through the stage of organisation of one or more classes lower in the scale, or, every animal begins its metamorphosis with the simplest organisation" (p. 57).
Thus the higher animals begin life as a kind of fluid animal jelly which resembles the substance of a polyp; the young mammal, like the lower Vertebrates, has only a simple circulation, and, like them, lives in water (the amniotic fluid); the frog is first like a worm, then develops gills and becomes like a fish (p. 57). In his work on the anatomy of the brain,[147] Tiedemann established the homology of the optic lobes in birds by comparing them with fœtal corpora quadrigemina in man (see Serres, Ann. Sci. nat., xii., p. 112).
J. F. Meckel, in 1811, devoted a long essay to a detailed proof of the parallelism between the embryonic states of the higher animals and the permanent states of the lower animals. In a previous memoir in the same collection[148] (i., 1, 1808) he had made some comparisons of this kind in dealing with the development of the human fœtus; in this memoir (ii., 1, 1811) he brings together all the facts which seem to prove the parallelism.
His collection of facts is a very heterogeneous one; he mingles morphological with physiological analogies, and makes the most far-fetched comparisons between organs belonging to animals of the most diverse groups. He compares, for instance, the placenta with the gills of fish, of molluscs and of worms, homologising the cotyledons with the separate tufts of gills in Tethys, Scyllæa and Arenicola(p. 26). This is purely a physiological analogy. He compares the closed anus of the early human embryo with the permanent absence of an anus in Cœlentera, and the embryo's lack of teeth with the absence of teeth in many reptiles and fish, in birds, and in many Cetacea (p. 46).[149] These are merely chance resemblances of no morphological importance. He considers bladderworms as animals which have never escaped from their amnion, and Volvox as not having developed beyond the level of an egg (p. 7). He lays much stress upon likeness of shape and of relative size, comparing, for instance, the large multilobate liver of the human fœtus with the many-lobed liver of lower Vertebrates and of Invertebrates. In general he shows himself, in his comparisons, lacking in morphological insight.
His treatment of the vascular system affords perhaps the best example of his method (pp. 8-25). The simplest form of heart is the simple tubular organ in insects, and it is under this form that the heart first appears in the developing chick. The bent form of the embryonic heart recalls the heart of spiders; it lies at first free, as in the mollusc Anomia. The heart consists at first of one chamber only, recalling the one-chambered heart of Crustacea. A little later three chambers are developed, the auricle, ventricle, and aortic bulb; at this stage there is a resemblance to the heart of fish and amphibia. At the end of the fourth day the auricle becomes divided into two, affording a parallel with the adult heart of many reptiles.
In his large text-book of a somewhat later date, the System der vergleichenden Anatomie (i., 1821), he works out the idea again and gives to it a much wider theoretic sweep, hinting that the development of the individual is a repetition of the evolutionary history of the race. Meckel was a timid believer in evolution. He thought it quite possible that much of the variety of animal form was due to a process of evolution caused by forces inherent in the organism. "The transformations," he writes, "which have determined the most remarkable changes in the number and development of the instruments of organisation are incontestably much more the consequence of the tendency, inherent in organic matter, which leads it insensibly to rise to higher states of organisation, passing through a series of intermediate states."[150]
His final enunciation of the law of parallelism in this same volume shows that he considered the development of the individual to be due to the same forces that rule evolution. "The development of the individual organism obeys the same laws as the development of the whole animal series; that is to say, the higher animal, in its gradual evolution, essentially passes through the permanent organic stages which lie below it; a circumstance which allows us to assume a close analogy between the differences which exist between the diverse stages of development, and between each of the animal classes" (p. 514).
He was not, of course, able fully to prove his contention that the lower animals are the embryos of the higher, and we gather from the following passage that he could maintain it only in a somewhat modified form. "It is certain," he writes, "that if a given organ shows in the embryo of a higher animal a given form, identical with that shown throughout life by an animal belonging to a lower class, the embryo, in respect of this portion of its economy, belongs to the class in question" (p. 535). The embryo of a Vertebrate might at a certain stage of development, be called a mollusc, if for instance, it had the heart of a mollusc.
He admits, too, that the highest animal of all does not pass through in his development the entire animal series. But the embryo of man always and necessarily passes through many animal stages, at least as regards its single organs and organ-systems, and this is enough in Meckel's eyes to justify the law of parallelism (p. 535).
In his excellent discussion of teratology Meckel points out how the idea of parallelism throws light upon certain abnormalities which are found to be normal in other (lower) forms (p. 556).[151]
We may refer to one other statement of the law of parallelism—by K. G. Carus in his Lehrbuch der vergleichenden Anatomie (Leipzig, 1834). The standpoint is again that of Naturphilosophie. It is a general law of Nature, Carus thinks, that the higher formations include the lower; thus the animal includes the vegetable, for it possesses the "vegetative" as well as the "animal" organs. So it is, too, by a rational necessity that the development of a perfect animal repeats the series of antecedent formations.
As we have said, the main credit for the enunciation of the law of parallelism belongs to the German transcendental school; but the law owes much also to Serres, who, with Meckel, worked out its implications. It might for convenience, and in order to distinguish it from the laws later enunciated by von Baer and Haeckel, be called the law of Meckel-Serres.
Under the "theory of the repetition or multiplication of parts within the organism" may be included, first, generalisations on the serial homology of parts, and second, more or less confused attempts to demonstrate that the whole organisation is repeated in certain of the parts. The recognition of serial homologies constituted a real advance in morphology; the "philosophical" idea of the repetition of the whole in the parts led to many absurdities. It led Oken to assert that in the head the whole trunk is repeated, that the upper jaw corresponds to the arms, the lower to the legs, that in each jaw the same bony divisions exist as in the limbs, the teeth, for instance, corresponding to the claws (loc. cit., p. 408). It led him to distinguish "two animals" in every body—the cephalic and the sexual animal. Each of these has its own organs; thus "in the perfect animal there are two intestinal systems thoroughly distinct from each other, two intestines which belong to two different animals, the sexual and cephalic animal, or the plant and the animal" (p. 382). The intestine of the sexual animal is the large intestine; the lungs of the sexual animal are the kidneys, its glottis is the urethra, its mouth the anus. So, too, the mouth is the stomach of the head. On another line of thought the sternum is a ventral vertebral column. Limbs are connate ribs, the digits indicating the number of ribs included (cf. Dugès, supra, p. 88).
J. F. Meckel[152] discusses "homologies" of this kind in the thorough and pedestrian way so characteristic of him. Not only, he says, are the right and left halves of the body comparable with one another, but also the upper and the lower, the dividing line being drawn at the level of the diaphragm. The lumbar complex corresponds to the skull, the anus to the mouth, the urino-genital opening to the nasal opening; in general, the urino-genital system corresponds to the respiratory, the kidneys to the lungs, the ureters to bronchi, the testes and ovaries to the thymus (he had observed the physiological relation between the development of the thymus and the state of the genital organs), the prostate and the uterus to the thyroid gland, and the penis and clitoris to the tongue. The fore-limbs and girdle correspond in detail with the hind limbs and the pelvis—a point already worked out by Vicq d'Azyr; the dorsal and ventral halves of the body are likewise comparable in some respects, the sternum, for example, answering in the arrangement of its bones, muscles and arteries to the vertebral column. The skeleton of each member is in some respects a repetition of the vertebral column.
His brother, D. A. Meckel,[153] worked out an elaborate comparison between the alimentary canal and the genital organs, basing the legitimacy of the comparison upon early embryological relations and upon the state of things in Cœlentera, where genital and digestive organs occupy the same cavity. In his view the uterus corresponded to the stomach, the vagina to the œsophagus, the fallopian tubes to the intestine, and so on.
The vertebral theory of the skull took its origin from the same habit of thought. As part of the wider idea of the metameric repetition of parts it had some scientific worth, but the theory was pushed too far, and the facts were twisted to suit it. Among annulate animals the theory of repetition found ample scope; Oken was able to compare with justice the jaws of crabs and insects with their other limbs, as Savigny did later in a more scientific way. Among Vertebrates the application of the theory of serial repetition was not so obvious, except in the case of the vertebræ. Goethe seems to have been the first to hit upon the idea that the skull is composed of a number of vertebræ, serially homologous with those of the vertebral column. He tells us that the idea flashed into his mind when contemplating in the Jewish cemetery at Venice a dried sheep's skull. The discovery was made in 1790, but not published till 1820.[154]
The idea seems to have been taught by Kielmeyer, one of the earliest of the "philosophers of nature," but it was not published by him.
In a book (Cours d'Études médicales), published in 1803, Burdin assimilated the skull to the vertebral column.
Oken, in an inaugural dissertation (Programm) Ueber die Bedeutung der Schädelknochen,[155] published in 1807, gave to the theory its necessary development. Autenrieth, also in 1807,[156] distinguishing separate ganglia in the brain, was not far from the hypothesis that each of these ganglia must have its separate vertebra.
In 1808 Duméril read a paper to the Académie des Sciences in which he compared the skull to a gigantic vertebra, basing his hypothesis on the similarity existing between the crests and depressions on the hinder part of the skull and those on the posterior surfaces of the vertebræ.
After Oken's work the vertebral theory was taken up generally by both the German and the French anatomists. Spix published in 1815 a large volume on the skull, entitled Cephalogenesis, distinguishing (as Oken did at first) three cranial vertebræ. Bojanus in his Anatome testudinis europæae (1819), and in a series of papers in Isis (1817-1819, and 1821) established the existence of a fourth cranial vertebra, and this was accepted by Oken in the later editions of his Lehrbuch. Meckel and Carus among the Germans, de Blainville and E. Geoffroy among the French, contributed to the development of the theory. In England the theory was championed particularly by Richard Owen.
It was one thing to assert in a moment of inspiration that the skull was composed of modified vertebræ; it was quite another to demonstrate the relation of the separate bones of the skull to the supposed vertebræ. Upon this much uncertainty reigned; there was not even unanimity as to the number of vertebræ to be distinguished. Goethe found six vertebræ in the skull; Spix, and at first Oken, three only, Geoffroy seven; the accepted orthodox number seems to have been four (Bojanus, Oken, Owen).
As an example of the method of treatment adopted we may take Oken's matured account of the composition of the cranial vertebræ, as given in the English translation of his Lehrbuch. "To a perfect vertebra," he says, "belong at least five pieces, namely, the body, in front the two ribs, behind the two arches or spinous processes" (p. 370). In the cervical vertebræ the transverse processes represent the ribs. The skull consists of four vertebræ, the occipital, the parietal, the frontal and the nasal, or, named after the sense with which each is associated, the auditory, the lingual, the ocular and the olfactory. The "bodies" of these vertebræ are the body of the occipital (basioccipital), the two bodies of the sphenoid (basi- and pre-sphenoid), and the vomer. The transverse processes of each are the condyles of the occipitals (exoccipitals), the alæ of the two sphenoids (alisphenoids and orbitosphenoids) and the lateral surfaces of the vomer. The arches or spinous processes are the occipital crest, the parietals, the frontals, and the nasals.
The cranium is thus composed of four rings of bone, each composed of the typical elements of a vertebra.
The arbitrary nature of the comparison is obvious enough. As Cuvier pointed out in the posthumous edition of his Leçons, it is only the occipital segment that shows any real analogy with a vertebra—an analogy which Cuvier ascribed to similarity of function. He admitted a faint resemblance of the parietal segment to a vertebra:—"The body of the sphenoid does indeed look like a repetition of the basioccipital, but having a different function it takes on another form, especially above, by reason of its posterior clinoid apophyses."[157] He denied the resemblance of the frontal and nasal "vertebræ" to true vertebræ, pointing out that both parietals and frontals are bones specially developed for the purpose of roofing over and protecting the cerebrum.
A very curious development was given to the vertebral theory by K. G. Carus, who seems to have taken as his text a saying of Oken's, that the whole skeleton is only a repeated vertebra.[158] His system is worthy of some consideration, for he tries to work out a geometry of the skeleton.[159]
His method of deduction is a good example of pure Naturphilosophie. Life, he says, is the development of something determinate from something indeterminate. A finite indeterminate thing, that is, a liquid, must take a spherical form if it is to exist as an individual. Hence the sphere is the prototype of every organic body. Development takes place by antagonism, by polarity, typically by the division and multiplication of the sphere. In the course of development the sphere may change, by expansion into an egg-shaped body, or by contraction into a crystalline form, the changes due to expansion being typical of living things, those due to contraction being typical of dead. At the surface of the primitive living sphere is developed the protective dermatoskeleton, which naturally takes the shape of a hollow sphere; round the digestive cavity which is formed in the living sphere is developed the splanchnoskeleton; round the nervous system (which is, as it were, the animal within the animal) is developed the neuroskeleton. All skeletal formations belong to one or other of these systems.
Carus defines his aim to be the discovery of the inner law which presides over the formation of the skeleton throughout the animal kingdom; he desires to know "how such and such a formation is realised in virtue of the eternal laws of reason" (iii., p. 93). Here we touch the kernel of Naturphilosophie—the search for rational laws which are active in Nature; the discontent with merely empirical laws.
The thesis which Carus sustains is that all forms of skeleton, whether of dermatoskeleton, splanchnoskeleton, or neuroskeleton, can be deduced from the hollow sphere, which is the primary form of any skeleton whatsoever (p. 95). That means, put empirically, that every skeleton can be represented schematically by a number of hollow spheres, suitably modified in shape, and suitably arranged. The chief modification in shape exhibited by bones is one which is intermediate between the organic and the crystalline series of modifications of the sphere. The organic modifications are bounded by curved lines, the crystalline by straight; the intermediate partly by curved and partly by straight lines. They are the dicone (the shape of a diabolo) and the cylinder. These forms must necessarily be of importance for the skeleton, which is intermediate between the organic and the inorganic. "The dicone embodies the real significance of the bone," writes Carus. Each dicone and cylinder composing the skeleton is called by Carus a vertebra.
We may expect then all skeletons to be composed of spheres, cylinders and dicones in diverse arrangements. Nature being infinite, all the possible types of arrangement of these elements must exist in the test or skeleton of some animal, living, fossil, or to come (p. 127). One conceives easily what the main types of skeleton must be. In some animals, e.g., sea-urchins, the skeleton is a simple sphere; in others, e.g., starfish, secondary rows of spheres radiate out from a central sphere or ring; in annulate animals the skeleton consists of a row of partially fused spheres.
In Vertebrates the arrangement is more complex. There are first the protovertebral rings of the dermatoskeleton, these being principally the ribs, limb-girdles, and jaws. Round the central nervous system are developed the deutovertebral rings of the neuroskeleton (vertebræ in the ordinary sense). The apophyses and bodies of the vertebræ, and the bones of the members[160] are composed of columns of tritovertebræ, or vertebræ of the third order. Thus the whole vertebrate skeleton is a particular arrangement of vertebræ, which in their turn are modifications of the primary hollow sphere.
The German transcendentalists were more or less contemporary with E. Geoffroy, and no doubt influenced him, especially in his later years, as they certainly did his follower Serres. Oken indeed wrote, in a note[161] appended to Geoffroy's paper on the vertebral column of insects, that "Mr Geoffroy [sic] is without a doubt the first to introduce in France Naturphilosophie into comparative anatomy, that is to say, that philosophy one of whose doctrines it is to seek after the signification of organs in the scale of organised beings." This is, however, an exaggeration, for Geoffroy was primarily a morphologist, whereas the morphology of the German transcendentalists was only a side-issue of their Naturphilosophie.
Geoffroy, on his part, exercised some influence on the transcendentalists. He asserts[162] indeed that Spix got some of the ideas published in the Cephalogenesis (1815) from attending his course of lectures in 1809. It is certainly the case that Spix published before Geoffroy the view that the opercular bones are homologous with the ear-ossicles, adopting, however, a different homology for the separate bones.[163]
Some speculations seem to have been common to both schools—for instance, the law of Meckel-Serres, the vertebral theory of the skull, and the recognition of serial homology in the appendages of Arthropods (Savigny, Oken). Latreille and Dugès, as well as Serres, clearly show in their theoretical views the influence of Oken and the other transcendentalists. Geoffroy's principle of connections and law of compensation were recognised by some at least of the Germans.
But whatever his actual historical relations may have been with the German school, Geoffroy was vastly their superior in the matter of pure morphology. He alone brought to clear consciousness the principles on which a pure morphology could be based: the Germans were transcendental philosophers first, and morphologists after.
One understands from this how J. F. Meckel, who was in some ways the leading comparative anatomist in Germany at this time, could be at once a transcendentalist and an opponent of Geoffroy. Meckel had a curiously eclectic mind. A disciple of Cuvier, having studied in 1804-6 the rich collections at the Museum in Paris, the translator of Cuvier's Leçons d'anatomie comparée, he earned for himself the title of the "German Cuvier," partly through the publication of his comprehensive textbook (System der vergl. Anatomie, 5 vols.), partly by his extensive and many-sided research work, partly by his authoritative teaching. His System shows in almost every page of its theoretical part the influence of Cuvier; and it is through having assimilated Cuvier's teaching as to the importance of function that Meckel combats Geoffroy's law of connections, at least in its rigorous form. He submits that the connections of bones and muscles must change in relation to functional requirements. He rejects Geoffroy's theory of the vertebrate nature of Articulates. Generally throughout his work the functional point of view is well to the fore.
Yet at heart Meckel was a transcendentalist of the German school. His vagaries on the subject of "homologues" leave no doubt about that, and, in spite of Cuvier, he believed, though not very firmly, in the existence of one single type of structure.
A Cuverian by training, his lack of morphological sense threw him into the ranks of the transcendentalists, to whom perhaps he belonged by nature.
[141] For a full account, see Kohlbrugge, Zool. Annalen, xxxviii., 1911.
[142] Rede über das Verhältnis der organischen Kräfte, Stuttgart u. Tübingen, 1793 (1814). See Rádl, loc. cit., i., p. 261; ii., p. 57.
[143] Supplem. ad historiam embryonis, Tübingen, 1797.
[144] Lehrbuch der Naturphilosophie, Eng. trans., p. 491, 1847.
[145] Ueber Entwickelungsgeschichte der Thiere, i., p. xvii., 1828.
[146] Zoologie, Landshut, i., 1808.
[147] Anatomie u. Bildungsgeschichte des Gehirns im Fötus des Menschen, Nürnberg, 1816.
[148] Beyträge zur vergleichende Anatomie, Leipzig, i., 1808-9, ii., 1811-2.
[149] Cetacea were generally considered at this time to be mammals of low organisation.
[150] From the French trans., which appeared under the title Traité gén. d'Anat. comparée, i., p. 449, 1828.
[151] Cf. Geoffroy (supra, p. 70).
[152] Beyträge, ii., 2, 1812. Also in his System d. vergl. Anat., i., 1821.
[153] In J. F. Meckel's Beyträge, ii.
[154] Zur Morphologie, i., 2, p. 250, 1820; and ii., 2, pp. 122-4, 1824.
[155] See translation, giving the gist of this paper, in Huxley's Lectures on the Elements of Comparative Anatomy, pp. 282-6, London, 1864.
[156] Reil's Archiv. f. Physiol., vii., 1807.
[157] Leçons d'anatomie comparée, 3rd ed., Brussels reprint, i., p. 414, 1836.
[158] In his Programm, U. d. Bedeut. d. Schädelknochen, 1807.
[159] Traité élémentaire d'anatomie comparée (French trans.), vol. iii., Paris, 1835. First developed in his volume Von den Ur-Theilen des Knochen und Schalen-Gerustes, Leipzig, 1828.
[160] Dutrochet in 1821 had tried to prove that the bones of the members belong to the type of the vertebra—the dicone.
[161] Isis, pp. 552-9, 1820 (2).
[162] Mém. Mus. d'Hist. nat., ix., 1822.
[163] Cuvier and Valenciennes, Hist. nat. Poissons, i., p. 311, f.n.